01 Denali XL Bose

My 01 denali stereo has dropped all 4 speakers in the door. Only the tweeters in the side of the center console work and the back 2 speakers and the sub work. But I get NO sound at any of the 4 door speakers. Anyone know if this is a radio or amplifier problem? I have no way of verifying wich one it is. I hope someone outthere can direct me to wich one it might be. Thanks